Enhanced SMTP status code · RFC 3463 · IANA
5.5.0 Protocol problem
Something went wrong with the delivery protocol to the next server, with no more precise cause.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
Also seen as:
2.5.0,4.5.0 - Subject: X.5 · Protocol
Failures of the delivery protocol: implementation errors or an unreliable connection.
How 5.5.0 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
-
451 4.5.0
SMTP protocol violation.
The sending server did not follow the SMTP protocol (RFC 5321): the problem is in its software or configuration.
Microsoft 365 / Exchange Online
-
5.5.0
Requested action not taken: mailbox unavailable
The recipient's @hotmail.com or @outlook.com address does not exist.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026